Sixteen VLS cells can be a false economy

Is building a new frigate with only 16 VLS cells a serious design mistake?

Yes. At least 32 cells usually offer much more magazine depth and flexibility for relatively little additional system cost.

Quad-packing CAMM or Sea Sparrow can turn sixteen VLS cells into 64 defensive missiles, but filling every cell that way restricts the loadout. Thirty-two quad-packed cells could hold 128 missiles, or part of the larger bank could carry other weapon types. The recommendation in this answer is to allow at least thirty-two cells on a frigate of several thousand tons. The ship already needs weapon-control computers and support systems for the smaller battery. Additional launcher modules are therefore presented as a relatively modest cost for more magazine depth and flexibility.